1. Field of the Invention
The invention relates to protective relay apparatus and methods for protecting A.C. electrical power transmission lines.
2. Description of the Prior Art
Conventional distance relaying for protecting three-phase electrical power transmission lines detects line impedance changes at power frequency, i.e., 50 or 60 Hz. The filtering necessary to filter out fault transients limits the reaction time of this approach. Since increasing the speed of fault detection and the clearing of the fault improves system stability, it has been proposed that the initial changes in line voltage and line current caused by incident traveling waves generated by a fault be used to detect and clear faults faster than possible by using power frequency measurements.
One prior art approach using traveling waves plots the trajectory of the fault generated voltage and current deviations as a function of time on a deviation plane, with the voltage deviation being on the X-axis and the current deviation on the Y-axis. The trajectory of such a plot is made up of two components. The first component, which is responsive to the power frequency signals, defines an elliptical orbit, with CW rotation indicating a fault in the forward direction, and CCW rotation indicating a fault in the reverse direction. The second component, which is responsive to traveling wave activity, causes the underlying elliptical orbit to be distorted by a high frequency component which is not elliptical. By setting up suitable boundary thresholds in each quandrant, the direction of a fault relative to the relaying point may be determined by detecting the first boundary crossed by a fault generated trajectory. The trajectory starts from the X-axis. Thus, if the first boundary crossed is in the second or fourth quadrants, the orbit is CW and the fault is in the direction of the protected line, i.e., a forward fault. If the first boundary crossed is in the first or third quadrants, the orbit is CCW and the fault is a reverse direction fault. While this technique enables very quick directional determination, it possesses some practical problems which make it difficult to apply. For example, in order to prevent the generation of high frequency transients in a non-faulted phase due to mutual coupling with a faulted phase, as well as to minimize the effects of lightning impulses and high frequency transients caused by close-in faults, the deviation signals must be passed through a low pass filter having a cut-off of about 1 KHz. This filtering can reduce the initial magnitude of the trajectory such that it misses the boundary threshold in the entry quadrant, and it then crosses the boundary threshold in the next quadrant. When this happens, the detection indicates that the fault is in the opposite direction from its actual location. Also, since the elliptical trajectory crosses both forward and reverse boundary thresholds, it is not only essential that the trajectory cross a boundary in the entry quadrant, but it is also important that the first boundary crossing be latched or memorized, and subsequent boundary crossings be ignored. Thus, when multiple faults occur, such as during a lightning storm, it would be possible to recognize a fault in the reverse direction, latch it, block subsequent recognitions, and miss a closely following fault in the forward direction.
The power frequency voltage and current deviations, in addition to producing an elliptical orbit which indicates direction to the fault, also contain sufficient information relative to distance from the fault to enable a direct or zone 1 trip decision. The boundaries are set such that if the trajectory crosses a forward boundary it is known that the fault is within the protected zone. This zone 1 trip decision, however, is complicated by the fact that the position and shape of the elliptical component of the orbit is responsive to both the fault inception angle .gamma. and the source impedance. Thus, it is extremely difficult to set the boundary threshold for a direct trip. Also, the traveling wave transients cause an overreach beyond the intended zone of protection. Filtering to remove this overreaching effect can produce incorrect operation, as hereinbefore stated.